Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. The result of this union is the production of a zygote cell, or fertilized egg.
In IVF lab the process is different. Eggs (oocytes) are retrieved from patients by transvaginal aspiration directly from the ovarian follicles, approximately 36 hours after an injection of HCG. The follicular fluid is taken into the laboratory so that the eggs can be identified by the embryologists as oocytes mature within the developing follicle; they are surrounded by cumulus cells which participate in the nourishment of the egg. The egg and the cumulus are referred to as the oocyte-cumulus complex.
Immediately after retrieval, each complex is identified under a microscope and then transferred to a special solution that is specially designed to provide all of the nutrients and other substances necessary to maximize the likelihood of successful fertilization of eggs by the sperm.
The dishes containing the eggs are then placed into an incubator, so that the environmental conditions surrounding the eggs can be tightly controlled with regard to light, oxygen and carbon dioxide concentrations, as well as the pH and temperature. Shortly before egg retrieval, a semen sample is collected. The retrieved eggs are placed in a laboratory dish with the motile sperm, where fertilization takes place. The fertilized eggs develop from 3 to 5 days in a special culture medium in a controlled environment, and are then transferred to the woman's uterus for potential implantation and embryo development.